Soybean, Glycine max (L.) Merril, is a major economic crop worldwide and is a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106. (1989). The growing demand for low cholesterol and high fiber diets has also increased the importance of the soybean as a health food.
Soybean varieties grown in the United States have a narrow genetic base, originating from Chinese strains. Six introductions, ‘Mandarin,’ ‘Manchu,’ ‘Mandarin’ (Ottawa), “Richland,’ ‘AK’ (Harrow), and ‘Mukden,’ contributed nearly 70% of the germplasm represented in 136 cultivar releases. To date, modern-day cultivars can be traced back from these six Chinese soybean strains. In a study conducted by Cox et al., Crop Sci. 25:529-532 (1988), the soybean germplasm is comprised of 90% adapted materials, 9% unadapted, and only 1% exotic species. The genetic base of cultivated soybeans could be widened through the use of exotic species. In addition, exotic species may possess such key traits as disease, stress, and insect resistance.
The soybean aphid, Aphis glycines Matsumura, was identified as a new insect pest of soybeans in 2001 and has spread to over 21 states in the United States and three Canadian provinces by 2003 (Vennette et al. Ann Entomol Soc Am 97:217-226 (2004)). High yields are critical to a farmer's profit margin, but soybean aphids can cause yield losses of 50% or more (Wang et al., Plant Protect 20:12-13 (1994)). In addition to the decrease in yield, an increase in insecticide use can also decrease a farmer's profit margin. Over 7 million acres of soybean in the North Central United States were sprayed with insecticide to control soybean aphids in 2003; the estimated cost of the insecticide treatments was between $84 and $105 million in the North Central region alone in 2003 (Landis et al. NCR-125 Arthropod biological control: state reports for 2003; Li et al., Mol Breeding 19:25-34 (2007)).
Soybean aphids can directly damage the plant by removing significant amounts of water and nutrients, causing the leaves to yellow and wilt. Additionally, aphids excrete honeydew, a sugar-rich sticky substance, onto the leaves and plants. Honeydew often leads to the development of sooty mold, which affects photosynthesis and results in significant yield losses (Gomez et al., Environ Exp Bot 55: 77-86 (2006)). Soybean aphids vector a number of viruses that can stunt plant growth, distort leaves, cause mottling of leaves and stem, reduce pod numbers, and cause discoloration in the seed. Viruses transmitted via soybean aphids include soybean mosaic virus, yellow mosaic virus, tobacco etch virus, and tobacco vein mottling virus (Wang et al. Plant Dis 90: 920-926 (2006)).
Aphid resistance genes, such as Rag1, have been identified in the soybean variety Dowling and mapped to linkage group M (U.S. patent application Ser. No. 11/158,307, issued as U.S. Pat. No. 7,994,389, both of which are incorporated by reference herein in their entireties). Additionally, quantitative trait locus (QTL) associated with aphid resistance were identified in Plant Introduction (PI) 567598B and mapped linkage groups B2, D1b, J and K (PCT Patent Application No. PCT/US2006/019200, incorporated by reference herein in its entirety). Aphid resistance from Dowling and PI 567598B has been effective against specific populations of aphids in certain geographies, but the resistance has broken down in broader aphid populations and geographies.
The present inventors have discovered that various biotypes of soybean aphid exist and can now be classified according to the methods of the present invention. A biotype is typically described as a population of a species of organisms having differentiable physiologic characteristics, such as response to a native resistance gene or resistance locus. The soybean variety ‘Dowling’ was described in 2004 as being resistant to soybean aphids and containing the Rag1 QTL for resistance. Within two years, a biotype of soybean aphid in Ohio was reported to overcome Rag1 resistance (Kim et al. Crop Sci 48:923-928 (2008)). This soybean aphid has not been characterized and there are currently no reports on the number of soybean aphid biotypes in the United States.